If you can throw together an edited collection, that's a good way of getting well-known, as long as it doesn't take away from your monographs and reviewed articles. If you were an unknown theorist and could become the editor of a book about some new hot theoretical trend in the 80s or 90s, you could get a reputation without having actually produced much theory of your own.
The problem with edited collections, I've heard, is having to work with a variety of authors with different attitudes toward prose-style and deadlines. The advantage is that you can have a book in your name without having to write the thing yourself. It's more cost-effective than editing a journal, where you do more work over a longer period of time, but don't have a book in your name no matter how long you've been the editor.
Editing more than one collection before you have a monograph is a mistake, according to the conventional view (with which I agree). The collection might be even more valuable to the field than your monograph is, but you need to show how you cultivate your own garden.
Edited collections are like any other book in that they can be excellent or not so excellent, but the difference is that they are likely to be less consistent in quality than monographs, especially if they are proceedings of an event (a conference or symposium) that haven't been individually refereed.
A tenure committee might count an edited collection as the equivalent of about 3 articles. 1/ 1/2 for the introduction, 1 1/2 for the editing. If the collection is super visible or influential, however, then that adds to its value. An edited collection on a single author or narrow subject is not likely to be influential. A book that appears to be a reference book might be visible, but does not quite seem as original. This judgment would depend on whether it was seen as a guide for undergraduates (unprestigious) or a book that introduces a new way of envisioning an entire field (prestigious).